Because high-density recording of data by information equipment and video-acoustic equipment has recently been progressed as represented by a DVD (Digital Video Disk), higher rotational accuracy has also requested for motors used for these pieces of equipment. Therefore, higher rotational accuracy has also been requested for a bearing unit for supporting the rotary shaft of a motor.
Most conventional bearing units using oil-impregnated sintered bearings are constituted respectively with a bearing of which inner surface is machined into a complete circle and with a so-called journal bearing in which a completely-circular shaft rotates and slides. The oil-impregnated sintered bearing shows a preferable sliding characteristic because of the circulating action of oil referred to as a pump action or the wedge effect of oil both appearing when a shaft eccentrically rotates. However, the journal bearing has such a structural problem that no radius-directional pressure is produced unless the shaft is eccentric, posing a problem that whirling of a shaft increases and the rotational accuracy can hardly be secured.
To solve the problems of the oil-impregnated sintered bearing, the official gazette of Japanese Utility Model Application Laid-Open No. 61-101124 discloses a bearing in which a spiral groove is formed on the inner surface of the bearing to realize a high rotational accuracy by leading a lubricant to the inside according as a shaft rotates and to reduce energy loss. Moreover, machining of grooves is facilitated because the bearing is made of a sintered metal and cost-savings can be realized. In addition, the official gazettes of Japanese Patent Application Laid-Open Nos. 62-167921 and 62-167922 disclose a bearing having a combination of three or more circular-arc planes having a curvature larger than a circular-arc plane about the shaft core on the inner periphery of the bearing and an inner surface whose circular-arc plane is flat, in which friction loss can be reduced because the bearing line-contacts the shaft. Moreover, the bearing disclosed in the official gazette of Japanese Patent Application Laid-Open No. 5-115146 is an oil-impregnated sintered bearing substantially having a plurality of rectangular grooves on the inner surface, which improves the dynamic pressure function and reduces noises and abrasion by a simple structure.
As described above, each of the already-disclosed arts reduces a fluid resistance without increasing a clearance for supporting a shaft by forming a groove on the inner surface of a bearing correspondingly to increase the fluid resistance of a fluid lubricant due to decrease of the clearance or increase of revolution speed. Moreover, each of the arts improves the bearing rigidity of a bearing support portion and the rotational accuracy by the dynamic pressure effect of a groove portion in accordance with the decrease of the fluid resistance.
The above structure makes it possible to obtain the effect for holding a shaft at the center by the dynamic pressure mechanism of a bearing. However, the structure has problems that the pressure nearby both ends of the bearing lowers which is important to control the whirling of the bearing because a lubricant leaks to the outside of the bearing sliding surface, the shaft whirling control effect is insufficient, and the rotational accuracy cannot be easily secured.
Moreover, as described above, because the lubricant nearby both ends of a bearing leaks to the outside of the bearing sliding surface in the case of a conventional bearing, the effect for holding a shaft at the center is also decreased. Furthermore, because it is requested to decrease a motor in size and thickness as equipment is downsized, when the length of a bearing is decreased, the influence of decreased bearing length particularly remarkably appears. In the case of examples in which characteristics of a journal bearing are studied in detail, some research results are reported that, when the ratio of a bearing length to a bearing diameter becomes 1, the dynamic pressure effect of a bearing is decreased up to approximately 1/2 when compared to the case in which the bearing is long. In this case, it is difficult to control shaft whirling and moreover, it is difficult to keep a necessary performance also from the viewpoint of the reliability of a bearing.